The present invention relates to a heat treatment apparatus which can conduct both normal pressure high temperature processing and low pressure processing.
In the fabrication of semiconductor products, objects to be treated, such as semiconductor devices, are subjected to a required heat treatment in a uniform thermal state. On the surfaces of the objects to be treated thin films are formed, heat diffusion is conducted, etc. The heat treatment apparatus is used for these purposes.
Heat treatment apparatus is known for normal pressure high temperature processing for heat treatment at, e.g., a substantially atmospheric pressure and a high temperature of, e.g., about 1000.degree. C., and a low-pressure heat treatment apparatus for heat treatment at a low pressure of, e.g., some Torr pressure at about 800.degree. C. A heat treatment apparatus is selected on the basis of the kind of heat treatment to be conducted on particular types of semiconductor wafers. A normal pressure high temperature heat treatment apparatus is sometimes followed by a low-pressure heat treatment apparatus, which requires moving the wafers. The reverse processing thereof is also conducted.
Such processing will be explained by means of, e.g., a low-pressure heat treatment apparatus. FIG. 13 shows a vertical sectional view of the conventional low-pressure heat treatment apparatus. A heating furnace 2 comprises a processing vessel 8 including an inner tube 4 of quartz having the lower end opened, and an outer tube 6 of quartz disposed concentrically with the inner tube. A heater 10 is wound around the outside of the outer tube 6.
A wafer boat 12 of, e.g., quartz is accommodated in the processing vessel 8, and is loaded and unloaded through the bottom into and out of the processing vessel 8. A number of objects to be processed, e.g., semiconductor wafers W are held on the boat 12 longitudinally at a set pitch.
A manifold 14 of stainless steel is connected to the bottom of the processing vessel 8. To be specific, a bottom flange 6A of the outer tube 6 is air-tightly connected through an O-ring 16 to a top flange 14A of the manifold 14, and the lower end of the inner tube 4 is removably set on an annular step 14B provided on the middle part of the manifold 14. A gas feed port 18 for introducing a processing gas into the processing vessel 8, and a gas exhaust port 20 for exhausting a gas in the processing vessel out of the apparatus are provided in the manifold 14.
In the bottom opening of the manifold 14 there is air-tightly provided, through an O-ring 28, a stainless steel cap 26 mounted on an arm 24 of lift means, such as an elevator or others. The wafer boat 12 is mounted on the cap 26 through a quartz heat insulating cylinder 30.
A rotary shaft journalled by a bearing 31 is inserted through the cap 26, and the heat insulating cylinder 30 is rotatably supported on the upper end of the rotary shaft.
Near the O-rings 16, 28, there are provided cooling means which prevent the O-rings 16, 28 from melting due to a high temperature of 800.degree. C. of the processing vessel.
When a heat treatment is conducted in such low-pressure heat treatment apparatus, a processing gas is fed through the gas feed port 18 into the interior of the processing vessel 8 maintained at a processing temperature of about 800.degree. C. and in a vacuum state of, e.g., 1 Torr. The introduced processing gas ascends in contact with a wafer region, processing the wafers W, and descends between the inner tube 4 and the outer tube 6 to be discharged outside the processing vessel 8 through the gas exhaust port 20.
In such conventional low-pressure heat treatment apparatus, a required heat treatment, e.g., thin film deposition, is followed by heat diffusion. Processed wafers are transferred into a different normal pressure high temperature heat treatment vessel to conduct the required heat treatment on the wafers.
It takes time to transfer the wafers, and throughputs are lowered. When the wafers are unloaded from the heating furnace for transfer, they are exposed to an atmosphere in a clean room. Particles can stay on the wafers and contaminate the wafers. These are problems with the conventional low-pressure heat treatment apparatus.
Furthermore, present practices essentially require two heat treatment apparatuses to conduct necessary heat treatments on wafers. This has added to equipment costs.
To solve these problems, it can be proposed to use one apparatus both as a low pressure heat treatment apparatus and a normal pressure high temperature heat treatment apparatus.
But when normal pressure high temperature processing is conducted in a low pressure heat treatment apparatus, the processing temperature is raised from about 800.degree. C. for a low pressure to about 1000.degree. C. at the normal pressure. Large amounts of corrosive gases, such as HCl, etc. corrode the stainless steel of the manifold 14.
When low pressure processing is conducted in a normal pressure high temperature heat treatment apparatus, the latter apparatus, which is intended for processing at a normal pressure, at around atmospheric pressure, uses an exhaust seal as a seal structure. It is a problem that the seal is easily broken at a low pressure vacuum state of about 1 Torr.
Heat treatment apparatus using, as processing gases, e.g., oxygen (O.sub.2) and corrosive hydrogen chloride (HCl), is substantially free from one problem with regard to the metal members at the lower interior part of the inner tube. Dry hydrogen chloride is relatively inert, and the processing gases flow in the reaction tube (specifically the inner tube) of the processing vessel from below to above. Thus, the apparatus disclosed in, e.g., Japanese Patent Laid-Open Publication (KOKAI) No. 138730/1990 does not much require corrosion preventive means for the metal members at the lower interior of the inner tube, as does heat treatment apparatus in which processing gases flow horizontally through the reaction tube.
But it is often the case with this heat treatment apparatus that exhaust gas after processing, e.g., hydrogen chloride takes in moisture and becomes very corrosive. In this case, the corrosive exhaust gas (containing corrosive products) descends between the inner and the outer tubes of the reaction tube to be discharged. The manifold becomes corroded when the processing gases pass between the lower ends of the inner and the outer tubes of the reaction tube. This is a problem with this heat treatment apparatus.
As means for solving this corrosion problem, it can be proposed that the reaction tube have the lower end portions of the inner tube and the outer tubes in one-piece. But a problem with this structure is that such a reaction tube is difficult to manufacture, clean and handle.
In the fabrication of semiconductor wafers, various heat treatment apparatuses for CVD, oxidation, diffusion, annealing, etc. are used. For example, a heat treatment apparatus (hereinafter also called vacuum heat treatment apparatus) for conducting processing in vacuum, such as low pressure CVD or others includes a processing gas feeding unit and an evacuation unit connected to the processing chamber for accommodating objects to be processed. A heat treatment apparatus (hereinafter also called ordinary pressure processing apparatus) for conducting processing at the ordinary pressure (atmospheric pressure) includes an ordinary pressure exhaust unit in place of the evacuation unit of the vacuum heat treatment apparatus connected to the processing chamber. The ordinary pressure heat treatment apparatus uses, in some cases, corrosive processing gases, e.g., hydrogen chloride (HCl).
The vacuum heat treatment apparatus and the ordinary pressure heat treatment apparatus simply differ from each other roughly in the exhaust unit. The exhaust unit includes both the evacuation unit and the ordinary exhaust unit, whereby a combined heat treatment apparatus can be provided. This combined heat treatment apparatus can achieve lower equipment costs, and it does not require semiconductor wafers to be transferred to a different heat treatment apparatus for each heat treatment, whereby various heat treatments can be continuously conducted. Accordingly higher quality and throughputs of semiconductor wafers can be achieved.
But the combined heat treatment apparatus has to include in the ordinary exhaust unit a corrosion resistant valve, e.g., a ball valve or others of, e.g., Teflon (registered trademark). Generally such valve is inferior in air-tightness. When the evacuation unit is operated for processing, exhaust gas particles flow reversely from the ordinary pressure exhaust unit, and if the exhaust gas, etc. have residual corrosive components, they may corrode machines and tools of the evacuation unit. Thus it has been difficult to realize a heat treatment apparatus which can conduct processing both in vacuum and at ordinary pressure.